The present invention relates to a method for controlling the operation of a perforated or screened belt press for filtering slurry, particularly sludge, with two superimposed perforated belts, a straining or filtering zone being formed through the lower belt and following on thereto a pressing zone is formed through the two belts, the drive for the perforated belts, the inflow of slurry, the flocculant addition, the belt tension and the belt nip or squeezing pressure being setable and/or regulatable.
Numerous different methods are known to make it possible to set and also regulate the operating conditions of a perforated belt press. In one known construction (German Patent No. 29 22 988), two independent control loops, namely a sludge throughput control loop with a regulatable sludge pump, a level sensor and a flow quantity meter on the one hand and a flocculant control loop with a regulatable flocculant pump, a sensor for the drainage state of the filter cake by measuring the lateral discharge thereof in the pressing zone and a flocculant quantity meter on the other are provided. If a particular perforated belt speed is set, the sludge throughput and flocculant inflow are regulated by these two control loops. The quality of this regulation is dependent on the measurement of the drainage state, i.e. the shear strength of the filter cake, but this can suddenly change and therefore lead to an incorrect actual value measurement. It is therefore difficult to achieve an optimization of the control of the operation of the perforated belt press.
Whereas in this known construction, there is no automatic adaptation to different slurries with variable drainage characteristics, this is sought in a further known construction (DE-OS No. 33 00 249). Hereagain independent control loops are provided and for the flocculant control loop the freely outflowing filtrate quantity in the straining or filtering zone is used as the reference input. This admittedly replaces the difficultly measurable shear strength of the filter cake by a continuously varying and therefore more easily measurable actual value. However, in actual fact the control quantity for the flocculant control loop is merely a different actual or test value, namely that of the filtrate in place of that of the filter cake. However, as soon as the slurry composition changes, it is also necessary to modify the desired value of the flocculant control loop if the perforated belt press is to be operated in an optimum manner. This optimization is to be achieved by taking account of additional parameters, e.g. the consistency of the slurry, but this makes the regulation much more complicated and costly.